1. Field of the Invention
The present invention relates to a charge control circuit and a method of adjusting a voltage detecting the full charge of a secondary battery in the charge control circuit.
2. Description of the Related Art
A lithium-ion secondary battery has a remarkably large charge capacity compared to a nickel-cadmium battery or a nickel-hydrogen battery. It is used widely for various uses such as batteries in laptop PCs and mobile phones utilizing such a characteristic. Similarly to a nickel-cadmium battery and a nickel-hydrogen battery, a lithium-ion secondary battery can not detect its full charge by detecting the peak voltage of the battery while being charged or a ΔV voltage drop from the peak voltage. Therefore, a lithium-ion secondary battery detects its full charge by detecting its battery voltage precisely. Thus, when the battery voltage has reached a predetermined voltage, it is determined that the battery is fully charged and the charging is terminated.
In a charge control circuit targeting such a lithium-ion secondary battery, a voltage being set for detecting the full charge status (hereinafter, “full charge detection voltage”) is fixed (see, for example, Japanese Patent Application Laid-open Publication No. Hei11-332121).
Since the full charge detection voltage is fixed, it is not possible to fine-adjust a full charge detection voltage in order to make the voltage suitable for the charge level of each type of battery and of each respective battery.
For example, a full charge detection voltage that does not degrade the performance of a secondary battery is a full charge detection voltage at which the secondary battery is maintained such that the battery is not over-charged and not over-discharged. In this way, the full charge detection voltage that does not degrade the battery performance differs by the type of the secondary battery and is not always constant. For example, it is most suitable for a lithium-ion secondary battery to have its full charge detection voltage set at a voltage that does not degrade its performance, ranging 4.1–4.2V/cell.